X-ray cameras are commonly employed in X-ray imaging systems such as X-ray microscopy, these systems being commonly used in such fields as crystallography or microanalysis. X-ray sensitive pick-up tubes employed in such cameras are similar to ordinary television camera tubes, such as fiber optic Vidicon tubes, except that a phosphor coating is added to the front of the input face of the tube. The phosphor coating is used because the coating causes a visible fluorescence in the presence of X-rays, whereas ordinary television camera tubes are insensitive to X-rays and thus cannot directly record X-ray images. The fluorescence produced by the fluorescent coating is visible light which is then sensed by the tube in the same manner as any other visible light image would be recorded.
Due to variations and fluctuations in the intensity of X-rays produced by X-ray sources, adjustments must be made to the bias and sensitivity range of the tube so that the resulting video image has a suitable range of intensities. A typical adjustment involves the placing of an opaque (e.g. lead) mask over the phosphor coating to eliminate fluorescence in a chosen region of the image frame, and establishing what is known in the art as a "black level reference." Unfortunately, the use of such a mask to establish this reference is often a tedious and difficult task. Care must be taken to align the mask with the rest of the optical elements in the X-ray optical path. A difficulty is that an X-ray source must be turned on for some aspects of the adjustment and calibration (e.g. fine tuning), and because of the danger of over-exposure to X-ray radiation, operators must be shielded from such sources, and from the tubes, during these alignment operations. This complicates both the manufacture and field service of instruments using these devices.